1. Field of the Invention
The present invention relates generally to a mobile communication system supporting a packet data service, and in particular, to an apparatus and method for transmitting and receiving control information related to a packet data channel (PDCH) for carrying packet data.
2. Description of the Related Art
A typical mobile communication system provides only a voice service to a mobile station (MS). However, with the development of the communication technology and at the request of users, researches have been carried out on a mobile communication system for supporting not only the voice service but also a data service for image communication and various Internet-related capabilities. A base station (BS) supporting both the voice and data services commonly classifies channels into circuit-based radio channels for the voice service and packet-based radio channels for the data service, and assigns the remaining radio resources except the radio resources used for the packet channels to packet channels. Here, the “radio resources” refer to transmission power, the number of Walsh codes, and a transmission period.
A CDMA2000 (Code Division Multiple Access 2000) 1xEV-DV (Evolution in Data and Voice) system, proposed to support both the voice service and high-speed packet data service, employs time division multiplexing (TDM) in order to transmit packet data to a plurality of users over a limited number of packet channels. A base station supporting the time division multiplexing assigns the entire power and Walsh codes available for packet data transmission to one mobile station for a particular time period. The base station selects mobile stations, to which it will assign radio resources every time period, by scheduling.
All mobile stations desiring to receive a data service from the base station must periodically report quality information of the radio channels to the base station. The base station then selects an optimal mobile station every time period, taking into account the radio channel quality information received from the mobile stations and an amount of accumulated data to be transmitted to the mobile stations so that the selected mobile station can maximize its data throughput while maintaining a data rate.
In the 1xEV-DV system, the number of Walsh codes available for packet data service is a maximum of 28, and the minimum time unit for transmitting packet data is 1.25 ms, which is called a “time slot (TS).” Actually, a time period for transmitting unit packet data becomes one of 1 TS, 2 TSs, 4 TSs, and 8 TSs, and data with a predetermined size is transmitted for this time period. The data with a predetermined size is called “encoder packet,” since it is encoded at once by an encoder in a base station transmitter.
A size of the encoder packet that can be transmitted for a time period is defined as 384, 768, 1,536, 2,340, 3,072 and 3,840 bits. The encoder packet undergoes encoding, interleaving and modulation by a base station transmitter, and then spread by at least one Walsh code before being transmitted over a packet data channel (PDCH). Here, a data rate is determined according to the encoder packet size and transmission time period.
A code rate and a modulation order (or modulation technique) used by a base station to transmit packet data are determined according to the encoder packet size and the number of Walsh codes, in order to obtain optimal data throughput. Information on the encoder packet size and the number of Walsh codes (hereinafter, referred to as “control information”) is transmitted to a mobile station over a separate packet data control channel different from the packet data channel. The mobile station then determines a modulation order and a code rate used for transmission of the packet data channel based on the control information, and receives packet data over the packet data channel.
Table 1 illustrates a frame format of control information transmitted over a packet data control channel (PDCCH).
TABLE 1FieldLength in BitsMAC ID8ARQ Channel ID2Encoder Packet Size3Subpacket ID2Total15
Describing information fields shown in Table 1, a MAC ID (Medium Access Control Identifier) field, indicating a unique identifier assigned to a mobile station desiring to receive a packet data service, is used to identify a mobile station that intends to transmit control information. An ARQ (Automatic Repeat Request) Channel ID field is used to identify a maximum of 4 data packets that can be simultaneously transmitted to one mobile station. An Encoder Packet Size field, indicating a size of transmission packet data, is assigned 3 bits to identify the 6 predefined sizes of 384, 768, 1,536, 2,340, 3,072, and 3,840 bits. A Subpacket ID field is used to identify a format of coded symbols to be used during initial transmission and retransmission.
The number of Walsh codes is represented using all fields except the MAC ID fields at predetermined periods or each time the number of Walsh codes is changed, and the MAC ID is set to ‘000000’. If MAC ID is set to ‘000000’, all mobile stations acquire information on the number of Walsh codes. In addition, a transmission time period (or the number of time slots) of packet data is equal to the transmission time period of a control information transmitted on a packet data control channel.
FIG. 1 illustrates a structure of a base station transmitter for transmitting control information over a packet data control channel (PDCCH) according to the prior art. Referring to FIG. 1, an 8-bit error correction code is added by an error correction code adder 110 to the 15-bit control information (MAC ID, ARQ Channel ID, Encoder Packet Size and Subpacket ID) illustrated in Table 1, and then 8 tail bits are added to the error correction code-added control information by a tail bit adder 120, for convergence into a specified state during decoding. A convolutional encoder 130 encodes 29 output bits from the tail bit adder 120 at a code rate R determined based on the control information, and outputs coded symbols. For example, if the control information is transmitted over 1 TS (N=1), the code rate becomes R=½. If the control information is transmitted over 2 TSs (N=2) or 4 TSs (N=4), the code rate becomes R=¼.
The coded symbols output from the convolutional encoder 130 are properly repeated by a symbol repeater 140 according to the transmission time period of the control information. That is, if the control information is transmitted over 1 TS or 2 TSs (N=1 or 2), the input coded symbols are not repeated, and if the control information is transmitted over 4 TSs (N=4), the input coded symbols are repeated once. A symbol puncturer 150 punctures some of the repeated coded symbols according to a predetermined puncturing pattern, and outputs the remaining non-punctured symbols.
An interleaver 160 sets a size of its interleaving memory according to the number of symbols received from the symbol puncturer 150, and interleaves the received symbols using the interleaving memory. A modulator 170 modulates the interleaved symbols according to a predetermined modulation order (e.g., QPSK (Quadrature Phase Shift Keying)) and outputs modulation (or modulated) symbols. A Walsh spreader 180 spreads the modulation symbols output from the modulator 170 with a 64-chip length Walsh code assigned to the packet data control channel. The Walsh code assigned to the packet data control channel is distinguishable from Walsh codes assigned to packet data channels. Although it is not shown, the output of the Walsh spreader 180, together with an output of a packet data channel transmitter, are converted into an RF (Radio Frequency) band signal through spreading by a PN (Pseudo-random Noise) code and baseband filtering, and then transmitted through an antenna.
A mobile station can acquire information on an encoder packet size, a transmission time period and the number of Walsh codes related to a packet data channel, from control information received over a packet data control channel, determine a code rate and a modulation order used by a base station for transmission of the packet data channel, based on the acquired information and can decode a received data on packet data channel.
In such a mobile communication system, a base station uses a mapping table representing a mapping relationship between a code rate and a modulation order based on the number of Walsh codes and the number of transmission time slots for an encoder packet size, in order to provide information on a code rate and a modulation order to a mobile station. FIG. 2 illustrates an portion of a mapping table for a 3,840-bit encoder packet size. In a Modulation Order column of the mapping table, “2” represents QPSK (Quadrature Phase Shift Keying), “3” represents 8-PSK (8-ary Phase Shirt Keying), and “4” represents 16-QAM (16-ary Quadrature Amplitude Modulator).
Referring to FIG. 2, when a base station transmits a 3,840-bit size encoder packet (or coded packet) according to a modulation order of 16-QAM and an effective code rate of 0.625 and the base station uses 4 Walsh codes during an 8-TS transmission time period. In this case, the base station transmits control information representing the 3,840-bit encoder packet, the 4 Walsh codes and the 8-TS transmission time period over a packet data control channel, by consulting the previously stored mapping table of FIG. 2. A mobile station then receives the control information over the packet data control channel, and determines that the modulation order of 16-QAM and the effective code rate of 0.625 were used for the packet data channel, by consulting the mapping table of FIG. 2 corresponding to the 3,840-bit size among mapping tables previously stored therein.
As described above, in the prior art, the base station and the mobile station require a plurality of mapping tables according to the encoder packet sizes. That is, the base station and the mobile station must previously store mapping tables of FIG. 2 for the individual encoder packet sizes, before they start data communication. In order to store the mapping tables, larger memory capacity is required. As a result, the base station and the mobile station must increase their required memory capacity. The increase in memory capacity may be an insignificant matter in the base station, which has no limitation on the memory capacity. However, in the case of the mobile station, the increase in the required memory capacity is disadvantageous when minimization and reduction in power consumption and cost of the mobile station is desired.
In addition, when a mobile station sends a retransmission request for received packet data due to an error that occurred in the received packet data, a base station may select for retransmission a modulation order and a code rate different from those used at initial transmission in order to increase reception efficiency of the mobile station. However, since the mapping table defines possible modulation order and code rate according to the number of Walsh codes and a transmission time period, the base station has a limitation in adaptively selecting a modulation order and a code rate according to a radio channel condition. Thus, to support more various combinations of the modulation orders and code rates, it is necessary to increase a size of the mapping table. However, the increase in size of the mapping table causes an increase in necessary memory capacity of the base station and the mobile station.